1. Field of the Invention
This invention relates to a method of and apparatus for separating magnetisable particles from a fluid in which they are suspended.
2. Description of the Prior Art
The types of apparatus which may be used for performing such a separation each comprise a matrix of ferromagnetic filamentary material which is contained in a separating chamber of non-magnetic material provided with an inlet and an outlet to enable a fluid in which magnetisable particles are suspended to be passed through the separating chamber. The separating chamber is positioned in an in-homogeneous magnetic field, the intensity of which is of the order of 10,000 gauss or more, and the magnetisable particles suspended in the liquid are attracted to the filamentary material of the matrix and remain in the matrix until the filamentary material is substantially demagnetised, whereupon the retained magnetisable particles can be dislodged by a stream of fluid.
In one type of such apparatus, a separating chamber of generally cylindrical shape is surrounded by an electromagnet coil which is substantially co-axial with the chamber, and which generates a magnetic field which, in the region occupied by the chamber, is parallel to the longitudinal axis of the chamber. A mixture of the magnetisable particles and the fluid enters the chamber at one end and leaves at the opposite end, the arrangement being such that the flow through the separating chamber is generally parallel to the axis and therefore substantially parallel to the direction of the magnetic field. In the other type of such apparatus, a separating chamber of generally cylindrical shape is positioned between two electromagnet pole pieces and the mixture enters the chamber at one end and leaves at the opposite end, the arragement being such that the direction of the magnetic field is perpendicular to the general direction of flow of the mixture.
With the first type the orientation in the matrix of a filament of the filamentary material is such that the filament is perpendicular to the direction of flow and hence perpendicular to the direction of the magnetic field. Under these conditions a magnetisable particle is captured if it enters a sector of the cylindrical space surrounding the filament which is on the upstream side of the filament and which is defined approximately by two planes each of which passes through the longitudinal axis of the filaments and each of which makes an angle of 45.degree. with the direction of the magnetic field. Magnetisable particles accumulate on approximately that part of the cylindrical surface of the filament which subtends an angle of 90.degree. at the longitudinal axis of the filament, and few, if any, particles adhere to the remainder of the surface.
With the second type a filament of the filamentary material again lies in a direction perpendicular to the direction of flow of the mixture and to the direction of the magnetic field. This arrangement generally results in an accumulation of magnetisable particles on the two sides of a filament when viewed from the direction from which the stream of mixture approaches, but any accumulation of particles on the sides of a filament tends to be swept off by the flowing mixture. As the number of particles accumulated increases so the flow of mixture round the filament is subjected to a greater deflection and the local velocity of the mixture is therefore increased, thus increasing the tendency to sweep away accumulated magnetisable particles.
In addition to the two foregoing general types, there is known, from U.S. Pat. No. 3,375,925 in the name of Carpenter, an apparatus comprising a separating chamber which is annular and is mounted so as to be rotatable about its axis. One or more electromagnets are utilized to apply a magnetic field perpendicular to the axis of the chamber at one or more circumferential regions of the annular chamber. Disposed within the separating chamber are a plurality of movable induced pole pieces, for example helical rods resembling drill bits arranged substantially parallel to the axis of the chamber.
In use of the apparatus the separating chamber is rotated about its axis so that each circumferential region of the separating chamber passes successively between the field poles of the electromagnet(s), which is (or are) activated from a power source. A slurry of the material to be separated is introduced into the separating chamber in a direction substantially parallel to the axis of the separating chamber. Separation is effected by differential modification of the velocities of the magnetically susceptible material and the non-magnetically susceptible material within the slurry as the slurry passes in proximity to the induced pole pieces and the induced pole pieces pass through the applied magnetic field. The greater or lesser portion of the magnetically susceptible material will not be retained statically at any time on the surfaces of the induced pole pieces, and another portion will be statically retained on the surfaces of the induced pole pieces for one or more short periods of time, so that the passage of this material through the apparatus will be intermitent. The separation takes place by virtue of the fact that the induced pole pieces will have moved through a greater distance when the magnetically susceptible material emerges from the apparatus compared with when the non-magnetically susceptible material emerges from the apparatus. A third portion of magnetically susceptible material will be statically retained on the induced pole pieces and will need to be flushed out after passing out of the magnetic field.
Since the minimum diameter of helical rods disclosed in this specification appears to be between 3/8 inch and 1/2 inch, it is essential that these rods should be provided with surface discontinuities (i.e. the helical grooves) in order to provide the necessary inhomogeneity of the magnetic field within the separating chamber.